1. Field of the Invention
The present invention relates to a method of controlling a fuel cell system including a fuel cell and a load driven by electric power outputted from the fuel cell.
2. Description of the Related Art
Conventionally, techniques of warming up a fuel cell utilized in a fuel cell vehicle or the like has been proposed (U.S. Patent Application Publication No. 2010/0047630 (hereinafter referred to as “US 2010/0047630 A1”). In US 2010/0047630 A1, the output voltage of the fuel cell (FC) is set to be equal to or larger than the minimum drive voltage of a load power source (see “Abstract” therein). More specifically, it is determined whether warm-up is necessary or not based on the temperature of the FC (S2 of FIG. 4 therein). If warm-up is necessary (S2: YES), it is confirmed whether or not there is any demand for the motor output (S3). If there is no demand for the motor output (S3: NO), a first low-efficiency operation is performed (S4 of FIG. 3). In the first low-efficiency operation, the output voltage of the FC is relatively low. If there is any demand for the motor output (S3: YES), a second low-efficiency operation is performed (S5 of FIG. 3). In the second low-efficiency operation, the output voltage of the FC is relatively high.
The first low-efficiency operation is used in the case where there is no demand for output from a traction motor (M3), and the second low-efficiency operation is used in the case where there is the output demand from the traction motor (paragraph [0044] therein). In the first low-efficiency operation, the output voltage of the FC is set to be less than the minimum drive voltage (Vlim) of the traction motor (paragraph [0045] therein). In the second low-efficiency operation, the FC current is calculated depending on the FC demand output, and the air stoichiometric ratio is changed depending on the calculated FC current. At this time, the FC output voltage is fixed at the minimum drive voltage (FIG. 5, paragraph [0046] therein).